Image forming apparatus, sheet finisher, and method for driving stacking tray

ABSTRACT

An image forming apparatus according to the present invention includes a printer, a stapler, a stacking tray, a driving mechanism, a detecting unit, a storage unit, and a controller. The printer prints image data on a sheet. The stapler staples a sheet bundle including the sheet. The stacking tray receives the sheet bundle at a standby position. The driving mechanism lifts and lowers the stacking tray. The detecting unit detects an upper surface of the sheet bundle on the stacking tray. The storage unit stores number of sheet bundles on the stacking tray. The controller controls the driving mechanism to increase a distance from the detecting unit to the stacking tray at the standby position according to an increase in the number stored in the storage unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from U.S. provisional application 61/015,182, filed on Dec. 19, 2007, and U.S. provisional application 61/016,944, filed on Dec. 27, 2007, the entire contents of each of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an image forming apparatus, a sheet finisher, and a method for driving a stacking tray, and, more particularly to an image forming apparatus, a sheet finisher, and a method for driving a stacking tray for stacking a sheet bundle to be subjected to stapling.

BACKGROUND

Recently, image forming apparatuses of an electrophotographic system such as a laser printer, a digital copying machine, and a laser facsimile include, for example, a sheet finisher disclosed in JP-2007-76893-A1 that staples a sheet bundle with a staple and sorts the sheet bundle.

As disclosed in, for example, JP-5-305786-A1, if a sheet bundle subjected to stapling is stacked on a stacking tray, in a sheet finisher, there is a technique for changing a lowering amount of the stacking tray more if a sheet bundle is discharged than if unstapled sheets are stacked on the stacking tray. The use of the technique can improve alignability during stacking on the stacking tray.

As disclosed in, for example, JP-11-310362-A1, there is a technique for providing, in a sheet finisher, an upper surface detection sensor for detecting an upper surface of a sheet bundle stacked on a stacking tray and lowering, if an upper surface of the stacking tray or the sheet bundle is detected, the stacking tray by a fixed amount.

However, if a large number of sheet bundles subjected to stapling are stacked on the stacking tray, a portion subjected to the stapling swells compared with a portion not subjected to the stapling. Therefore, an upper surface of the stacked sheet bundles cannot be normally detected. The problem is more conspicuous, for example, if sheet bundles subjected to the stapling in one place at one end of sheets are stacked than if sheet bundles subjected to the stapling in plural positions symmetrically are stacked.

If the sheet bundles subjected to the stapling in one place at one end of sheets are stacked as explained above, a portion subjected to the stapling swells. Therefore, even if an upper surface of the sheet bundles is detected near the center of the sheet bundles, if a large number of sheet bundles are stacked, a top surface of the stacked sheet bundles cannot be accurately detected.

Therefore, since an original upper surface is present in a position higher than a detectable upper surface and a swelling portion closes a discharge port of a processing tray, a jam occurs. If a large number of sheet bundles are stacked, the sheet bundles are unaligned because, for example, the sheet bundles bump against sheet bundles already stacked. The stacked sheet bundles fall from the stacking tray. This problem is more conspicuous if bundles of two sheets are stacked than if, for example, bundles of fifty sheets are stacked.

SUMMARY

The present invention has been devised in view of the circumstances and an object of the present invention is to provide an image forming apparatus, a sheet finisher, and a method for driving a stacking tray that can stack a large umber of sheet bundles subjected to stapling on the stacking tray and stack the sheet bundles with high alignability.

In order to attain the object, an image forming apparatus according to an aspect of the present invention includes: a printer configured to print image data on a sheet; a stapler configured to staple a sheet bundle, the sheet bundle including the sheet; a stacking tray configured to receive the sheet bundle at a standby position; a driving mechanism configured to lift and lower the stacking tray; a detecting unit configured to detect an upper surface of the sheet bundle on the stacking tray; a storage unit configured to store number of sheet bundles on the stacking tray; and a controller configured to control the driving mechanism to increase a distance from the detecting unit to the stacking tray at the standby position according to an increase in the number stored in the storage unit.

In order to attain the object, a sheet finisher according to another aspect of the present invention includes: a stapler configured to staple a sheet bundle, the sheet bundle including a sheet; a stacking tray configured to receive the sheet bundle at a standby position; a driving mechanism configured to lift and lower the stacking tray; a detecting unit configured to detect an upper surface of the sheet bundle on the stacking tray; a storage unit configured to store number of sheet bundles on the stacking tray; and a controller configured to control the driving mechanism to increase a distance from the detecting unit to the stacking tray at the standby position according to an increase in the number stored in the storage unit.

A method for driving a stacking tray according to still another aspect of the present invention includes acts of: stapling a sheet bundle, the sheet bundle including a sheet; and controlling driving of a stacking tray to increase a distance from a detecting unit that detects an upper surface of the sheet bundle on the stacking tray to the stacking tray at a standby position according to an increase in number of sheet bundles on the stacking tray.

DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a perspective view of an example of an external appearance of an image forming apparatus according to an embodiment of the present invention;

FIG. 2 is a diagram of a detailed configuration example of a sheet finisher of the image forming apparatus according to the embodiment;

FIG. 3 is a schematic diagram of a configuration of the sheet finisher;

FIG. 4 is a perspective view of the structure showing mainly inlet rollers, outlet rollers, a processing tray unit, and a stacking tray;

FIG. 5 is a perspective view of a first example of the structure of the processing tray unit;

FIG. 6 is a perspective view of a second example of the structure of the processing tray unit;

FIG. 7 is a perspective view of the processing tray unit viewed from an angle different from an angel shown in FIG. 5;

FIGS. 8 to 11 are schematic side views showing mainly a configuration example of a belt driving mechanism;

FIG. 12 is a perspective view of a main part of the sheet finisher viewed from the stacking tray side;

FIG. 13 is a side view of the main part of the sheet finisher;

FIG. 14 is a block diagram of a first example of functions of the image forming apparatus according to the embodiment;

FIG. 15 is a schematic diagram for explaining a function of a lifting controller;

FIGS. 16 to 20 are schematic diagrams for explaining a function of a lowering amount calculating unit;

FIG. 21 is a flowchart of a first example of acts of the image forming apparatus according to the embodiment;

FIG. 22 is a block diagram of a second example of the functions of the image forming apparatus according to the embodiment; and

FIG. 23 is a flowchart of a second example of the acts of the image forming apparatus according to the embodiment.

DETAILED DESCRIPTION

An image forming apparatus, a sheet finisher, and a method for driving a stacking tray according to an embodiment of the present invention are explained with reference to the accompanying drawings.

FIG. 1 is a diagram of an example of an external appearance of a copying machine (or an MFP) as a typical example of the image forming apparatus according to this embodiment.

An image forming apparatus 100 according to this embodiment is shown in FIG. 1. The image forming apparatus 100 includes an image forming apparatus main body (a printer, etc.) 1 and a sheet finisher 2. The image forming apparatus main body 1 includes a scanning unit 3, a printer 4, and a paper feeding unit 5.

The scanning unit 3 of the image forming apparatus main body 1 optically scans an original placed on a document table and an original received by an ADF (auto document feeder) and generates image data.

The printer 4 prints the image data on a sheet received from the paper feeding unit 5 using an electrophotographic system. The printer 4 has a control panel 6 with which a user performs various kinds of operation, and a display panel 7 on which various kinds of information are displayed.

The sheet finisher 2 is a device that applies post processing such as sorting and stapling by a staple to sheets P which the image forming apparatus main body 1 prints.

FIG. 2 is a diagram of a detailed configuration example of, in particular, the sheet finisher 2 of the image forming apparatus 100 according to this embodiment. FIG. 3 is a schematic diagram of a configuration of the sheet finisher 2.

The sheet finisher 2 adjacent to the image forming apparatus main body 1 has inlet rollers 11 a and 11 b on a side thereof. The inlet rollers 11 a and 11 b receive a printed sheet P discharged by the image forming apparatus main body 1. The inlet rollers 11 a and 11 b transport the received sheet P to outlet rollers 12 a and 12 b.

The sheet finisher 2 has a waiting tray 13 ahead of the outlet rollers 12 a and 12 b. The waiting tray 13 temporarily holds the sheet P which the outlet rollers 12 a and 12 b transport.

If a holding period passes and the waiting tray 13 is opened, the waiting tray 13 drops and feeds the sheet P temporarily held therein to a processing tray 14. On the processing tray 14, a plurality of the sheets P are placed one on top of another to form a sheet bundle B.

The sheet finisher 2 has the processing tray 14 to incline in an up to down direction. The sheet finisher 2 includes a stapler 19 ahead of a lower end side of the processing tray 14. The sheet finisher 2 includes a stacking tray (a movable tray) 23 which stacks the sheet bundle B ahead of an upper end side of the processing tray 14.

The sheet finisher 2 has a sheet guide 18 above the lower end side of the processing tray 14. The sheet guide 18 guides a trailing end of the sheet bundle B received by the processing tray 14 in the direction of the stapler 19.

The sheet finisher 2 has a jogger fence 16 on both sides of the processing tray 14. The jogger fence 16 laterally aligns the sheet P on the processing tray 14. The sheet finisher 2 has longitudinal alignment rollers 17 on a rear end side of the processing tray 14, and a paddle 15 on an upper side of the longitudinal alignment rollers 17. The paddle 15 and the longitudinal alignment rollers 17 strike a trailing end of the sheet P on the processing tray 14 against a rear stopper 26 and longitudinally align the sheet P.

The sheet finisher 2 sequentially guides the sheet P to the processing tray 14 through the waiting tray 13 and guides a plurality of the sheets P to the stapler 19 as the sheet bundle B. The stapler 19 staples the vicinity of a trailing end of the sheet bundle B with a staple.

In the sheet finisher 2, ejectors (hook members) 20 fixed to eject belts (toothed belts) 30 (shown in FIG. 5) and a bundle pawl (a transporting hook member) 21 a fixed to a bundle pawl belt (a transport or belt) 21 hook the trailing end of the sheet bundle B, and transport the sheet bundle B in the direction of the stacking tray 23 by the driving of the eject belts 30 and the bundle pawl belt 21. The sheet finisher 2 fixes push rods 25 on the opposite side of the ejectors 20 on the eject belts 30. The sheet finisher 2 projects the sheet bundle B in the direction of the stacking tray 23 with a lower surface thereof supported by the push rods 25 and discharges the sheet bundle B to the stacking tray 23 by discharging rollers 22.

The stacking tray 23 can stack a large number of the sheet bundles B. The stacking tray 23 gradually falls as a stacking amount of the sheet bundles B increases.

FIG. 4 is a perspective view of the structure showing mainly the inlet rollers 11, the outlet rollers 12, a processing tray unit 50, and the stacking tray 23. FIG. 5 is a perspective view of a first example of the structure of the processing tray unit 50.

As shown in FIG. 5, the processing tray 50 includes the processing tray 14 on which the sheet bundle B is placed. The sheet finisher 2 has the jogger fence 16 on both the sides of the processing tray 14. The jogger fence 16 aligns side ends of the sheet bundle B before stapling by being moved in the width direction. The two longitudinal alignment rollers 17 strike the trailing end of the sheet bundle B on the processing tray 14 against the rear stopper 26 to longitudinally align the sheet bundle B.

Post processing performed by the sheet finisher 2 includes sorting in addition to the stapling. In the sorting, with a sheet bundle to be laterally aligned and longitudinally aligned set as a sorting unit, the sheet finisher 2 alternately shifts the sheet bundle in the horizontal direction (left and right) for each sorting unit, and discharges the sheet bundle to the stacking tray 23. The jogger fence 16 shifts the sheet bundle in the horizontal direction.

The sheet finisher 2 discharges the sheet bundle B from the processing tray 14 to the stacking tray 23 via a belt driving mechanism 60 (shown in FIGS. 8 to 11) and the four discharging rollers 22. The sheet finisher 2 includes, in the belt driving mechanism 60, the bundle pawl belt 21, the bundle pawl 21 a fixed to the bundle pawl belt 21, the two eject belts 30 of both sides of the bundle pawl belt 21, and the ejectors 20 and the push rods 25 fixed to the eject belts 30.

FIG. 6 is a perspective view of a second example of the structure of the processing tray unit 50.

The processing tray unit 50 shown in FIG. 6 includes four push rods 25. Both outer sides of the two push rods 25 fixed to the eject belts 30 may have the same two push rods 25, respectively. The two push rods 25 on the outer side and the two push rods 25 on the inner side move in synchronization with each other. In the processing tray unit 50, the sheet bundle B can be supported in a range wide in the horizontal direction and projected in the direction of the stacking tray 23. Therefore, the sheet finisher 2 can perform stable transport with little positional deviation.

FIG. 7 is a perspective view of the processing tray unit 50 viewed from an angle different from an angle shown in FIG. 5.

The processing tray unit 50 has a motor 51 as a driving source for the bundle pawl belt 21 and the eject belts 30, and an electromagnetic clutch 52 for connecting and releasing transmission of driving force to the eject belts 30 in a lower part thereof.

FIGS. 8 to 11 are schematic side views of a configuration example showing mainly the belt driving mechanism 60.

As shown in FIGS. 8 to 11, the belt driving mechanism 60 includes the bundle pawl belt 21, the bundle pawl 21 a fixed to the bundle pawl belt 21, the two eject belts 30 of both the sides of the bundle pawl belt 21, and the ejectors 20 and the push rods 25 fixed to the eject belts 30.

FIG. 8 is a diagram of a state in which the eject belts 30 is in a home position (a first position) The ejectors 20 may be in a home position of the ejector 20. The push rods 25 fixed to the eject belts 30 may be in a home position of the push rods 25. The ejectors 20 stops in a state in which the ejectors 20 hook the trailing end of the sheet bundle B in a position substantially the same as the rear stopper 26.

If the ejectors 20 are in the home position, the belt driving mechanism 60 applies lateral alignment and longitudinal alignment to the sheet bundle B using the jogger fence 16 and the longitudinal alignment rollers 17, and staples by the stapler 19.

If the eject belts 30 are in the home position, the electromagnetic clutch 52 (shown in FIG. 7) are off and a toothed pulley 31 is disconnected from the rotation of the motor 51.

Even if the ejectors 20 are in the home position, the bundle pawl belt 21 continues the consecutive rotation in the counterclockwise direction. Immediately before the ejectors 20 start to move from the home position, the bundle pawl 21 a on the bundle pawl belt 21 moves, for example, in the vicinity below a toothed pulley 32 (a driven roller) located on the right side.

FIG. 9 is a diagram of a state after some time elapses after the electromagnetic clutch 52 is turned on in the state shown in FIG. 8. If the electromagnetic clutch 52 is turned on in the state shown in FIG. 8 (the state in which the eject belts 30 are in the home position and the bundle pawl 21 a is moving in the vicinity below the toothed pulley 32 (the driven roller), the toothed pulley 31 starts the rotation in the counterclockwise direction. The eject belts 30 (and the ejectors 20 and the push rods 25) start to move in the left direction in the figure (a forward path direction) according to the rotation of the toothed pulley 31. The ejectors 20 hook the trailing end side of the sheet bundle B. The push rods 25 support the leading end side of the sheet bundle B. The ejectors 20 transports the sheet bundle B to the stacking tray 23 side.

A torsion coil spring (an elastic member) inserted into one end of a pulley shaft of the toothed pulley 31 is wound up by the rotation of the toothed pulley 31 and elastic force is gradually accumulated.

On the other hand, the bundle pawl belt 21 continues the rotation. In the state shown in FIG. 9, the bundle pawl 21 a is approaching the ejectors 20 from behind the ejectors 20.

As shown in FIG. 10, when the bundle pawl 21 a overtakes the ejectors 20 and contacts an end of the sheet bundle B, the electromagnetic clutch 52 is turned off. For example, at a point in time when the ejectors 20 reached a second position, the electromagnetic clutch 52 turns off. The bundle pawl 21 a which the sheet bundle B contacts transports the sheet bundle B.

If the electromagnetic clutch 52 is turned off, as shown in FIG. 11, the toothed pulley 31 starts to rotate in the opposite direction (the clockwise direction) by the elastic force accumulated in the torsion coil spring. The ejectors 20 move to the home position in a backward path direction while increasing speed.

FIG. 12 is a perspective view of a main part of the sheet finisher 2 viewed from the stacking tray 23 side. FIG. 13 is a side view of the main part of the sheet finisher 2.

As shown in FIGS. 12 and 13, a driving mechanism for the stacking tray 23 of the sheet finisher 2 mainly includes a stacking tray motor 71 as a driving source, which is a DC (direct current) motor, a motor timing belt 72, a worm gear 73, gear and pulley 74 a and 74 b, a driving side pulley 75, a timing belt 76, an encoder sensor 77, and a driven side pulley.

The rotation driving of the stacking tray motor 71 transmits power to the worm gear 73, the gear and pulley 74 a and 74 b, and the driving side pulley 75 in this order via the motor timing belt 72. The rotation driving of the stacking tray motor 71 transmits the power to the timing belt 76 stretched between the stacking tray motor 71 and the driven side pulley.

The number of revolutions of the stacking tray motor 71, which is a driving amount of the stacking tray 23, is managed according to a count number of the encoder sensor 77 attached to the worm gear 73. If the stacking tray motor 71 is a pulse motor, the number of revolutions of the stacking tray motor 71 is managed according to a pulse number.

The sheet finisher 2 has upper surface detection sensors 78 below the four discharging rollers 22. In FIG. 13, the sheet finisher 2 has two upper surface detection sensors 78 below the two discharging rollers 22 in the center, respectively.

FIG. 14 is a block diagram of a first example of functions of the image forming apparatus according to this embodiment.

According to the execution of programs by a CPU (central processing unit) of the sheet finisher 2, the sheet finisher 2 functions as a sheet-information acquiring unit 101, a lifting controller 102, a lowering-amount calculating unit 103, and a lowering controller 104. The sheet finisher 2 may have the respective units 101 to 104 as hardware.

The sheet-information acquiring unit 101 has a function of acquiring, from the CPU of the image forming apparatus main body 1, sheet information of the sheet P discharged from the image forming apparatus main body 1 to the sheet finisher 2. The sheet information means a size of the sheet and sheet thickness of the sheet P. The sheet thickness may be calculated from the size of the sheet and sheet weight.

The lifting controller 102 has a function of, if a sheet bundle is discharged to the stacking tray 23 in the standby position, controlling the stacking tray motor 71 (shown in FIGS. 12 and 13) and lifting the stacking tray 23 until the upper surface detection sensors 78 detect the upper surface of the stacking tray 23 or the upper surface of the sheet bundle B.

FIG. 15 is a schematic diagram for explaining the function of the lifting controller 102.

As shown in FIG. 15, if the sheet bundle B is discharged from the processing tray 14 to the stacking tray 23 by the sheet pawl 21 a and the discharge rollers 22, the stacking tray 23 stays on standby in a position below the upper surface detection sensors 78 (a position of a broken line in FIG. 15) If the sheet bundle B is discharged to the stacking tray 23, the lifting controller 102 lifts the stacking tray 23 until the upper surface detection sensors 78 detect the upper surface of the sheet bundle B.

The lowering-amount calculating unit 103 shown in FIG. 14 has a function of calculating a lowering amount of the stacking tray 23. The lowering amount of the stacking ray 23 is a lowering amount of the upper surface of the sheet bundle B stacked on the stacking tray 23.

The lowering-amount calculating unit 103 calculates a lowering amount L of the stacking tray 23 such that a distance (I+L) obtained by adding up a distance I from a discharge port for discharging the sheet bundle B to the stacking tray 23 to detecting sections of the upper surface detection sensors 78 and a lowering amount L of the stacking tray 23 is equal to an optimum distance for discharging the sheet bundle B onto the stacking tray 23.

Lowering mount=L(fixed value)   (1)

The function of the lowering-amount calculating unit 103 is explained in detail with reference to FIGS. 16 to 19.

FIGS. 16 to 19 are schematic diagrams for explaining the function of the lowering-amount calculating unit 103.

It the sheet bundle B subjected to stapling, in particular, the sheet bundle B subjected to the stapling at a corner on one side is stacked on the stacking tray 23, as shown in FIG. 16, a difference occurs between the height z1 of a first end on a side subjected to the stapling and the height z2 of a second end on a side not subjected to the stapling.

If the difference occurs between the height z1 of the first end and the height z2 of the second end, as shown in FIG. 17, an error ze occurs between the height z1 of the one end as the actual height of the sheet bundle B and the height z3 of the sheet bundle B actually detected by the upper surface detection sensors 78. If the sheet bundle B has a sheet size large in sheet width, the error ze is larger because a distance y1 in the width direction from the upper surface detection sensors 78 to the sheet end is large compared with that in the case of a size with small sheet width.

Therefore, if the number of the sheet bundles B stacked on the stacking tray 23 gradually increases and the error ze gradually becomes larger, the error ze exceeds the distance (I+L) at certain timing. If the stacking tray 23 is lowered by the lowering amount L and the stacking tray 23 is put on standby despite the fact that the error ze exceeds the distance (I+L), because of the friction between the front end of the sheet bundle B discharged from the processing tray 14 next and the sheet bundle B to be stacked, the sheet bundles B are not stacked with high alignability. The sheet bundle B discharged from the processing tray 14 next pushes out the sheet bundle B to be stacked in an upper part of the stacking tray 23.

On the other hand, if the lowering amount L calculated by the above equation (1) is set to a value not exceeding the error ze, in particular, if a stacking amount of the stacking tray 23 is small, a falling distance of the discharged sheet bundle B is large. The discharged sheet bundle B is not stacked on the stacking tray 23 with high alignability.

Therefore, the lowering-amount calculating unit 103 calculates the error ze as a conversion error ze′ using the sheet information (the size and the thickness of the sheet P) acquired by the sheet-information acquiring unit 101 and information (the number of stacked sheet bundles, the thickness of a staple C, and the number of sheet bundles subjected to stapling) stored in a storage device such as a memory. The lowering-amount calculating unit 103 calculates a lowering amount (L+ze′) of the stacking tray 23 using a following equation (2):

Lowering amount=L+ze′  (2)

The lowering-amount calculating unit 103 calculates the conversion error ze′ according to a following equation (3) using the thickness of the staple C as a stapling material shown in FIG. 18:

ze′=(thickness of the staple C−(sheet thickness×number of sheet bundles subjected to stapling))×sheet width coefficient×number of stacked sheet bundles   (3)

The lowering-amount calculating unit 103 calculates the sheet width coefficient using a distance y1 (shown in FIG. 17) for each size of the sheet P. An example of the sheet width coefficient for each sheet size is shown in FIG. 19. When the sheet finisher 2 includes the plural upper surface detection sensors 78 as shown in FIG. 13, the lowering-amount calculating unit 103 only has to calculate the sheet width coefficient using the upper surface detection sensor 78 closest to a stapling position of the sheet bundle B.

If the conversion error ze′ is negative in the calculation by the above equation (3), the lowering-amount calculating unit 103 may regard the conversion error ze′ as zero. If the conversion error ze′ is negative, the lowering-amount calculating unit 103 calculates a lowering amount L shown on the left in FIG. 20. On the other hand, if the conversion error ze′ changes from negative to positive, the lowering-amount calculating unit 103 changes the lowering amount L to a lowering amount (L+ze′) shown on the right in FIG. 20.

The lowering controller 104 shown in FIG. 14 has a function of controlling the stacking tray motor 71 (shown in FIGS. 12 and 13) to lower the stacking tray 23 by a lowering amount calculated by the lowering-amount calculating unit 103.

FIG. 21 is a flowchart of a first example of acts of the image forming apparatus according to this embodiment.

First, the sheet finisher 2 of the image forming apparatus 100 acquires, from the CPU of the image forming apparatus main body 1 (shown in FIG. 16), sheet size information of a sheet discharged from the image forming apparatus main body 1 to the sheet finisher 2 (Act 1).

Subsequently, the sheet finisher 2 controls the stacking tray motor. 71 (shown in FIGS. 12 and 13) to lift the stacking tray 23 until the upper surface detection sensors 78 detect the upper surface of the stacking tray 23 or the upper surface of the sheet bundle B stacked on the stacking tray 23 (Act 2).

The sheet finisher 2 calculates the conversion error ze′ using the above equation (3) (Act 3). The sheet finisher 2 determines whether the conversion error ze′ calculated in Act 3 is positive (Act 4). If the sheet finisher 2 determines that the conversion error ze′ is positive (“YES” in Act 4), the sheet finisher 2 calculates, on the basis of the sheet information (the size and the thickness of the sheet P) acquired in Act 1 and the information (the number of stacked sheet bundles, the thickness of the staple C, and the number of sheet bundles subjected to stapling) stored in the storage device such as a memory, a lowering amount (L+ze′) of the stacking tray 23 using the above equation (2) (Act 5).

On the other hand, if the sheet finisher 2 determines that the conversion error ze′ is negative (“NO” in Act 4), the sheet finisher 2 regards the conversion error ze′ of the above equation (2) as zero and calculates the lowering amount L of the stacking tray 23 (Act 6).

The sheet finisher 2 controls the stacking tray motor 71 (shown in FIGS. 12 and 13) to lower the stacking tray 23 by the lowering amount calculated in Act 5 or Act 6 (Act 7). According to the lowering of the stacking tray 23 in Act 7, the stacking tray 23 stays on standby in a position after the lowering.

The sheet finisher 2 determines whether the discharge of the sheet bundle B to the stacking tray 23 should be finished (Act 8). If the sheet finisher 2 determines that the discharge of the sheet bundle B to the stacking tray 23 is finished (“YES” in Act 8), the sheet finisher 2 finishes the acts.

On the other hand, if the sheet finisher 2 determines that the discharge of the sheet bundle B to the stacking tray 23 is not finished (“NO” in Act 8), the sheet finisher 2 discharges the sheet bundle B from the processing tray 14 to the stacking tray 23 (Act 9). Subsequently, the sheet finisher 2 controls the stacking tray motor 71 to lift the stacking tray 23 until the upper surface detection sensors 78 detect the upper surface of the sheet bundle B stacked on the stacking tray 23 (Act 2).

FIG. 22 is a block diagram of a second example of the functions of the image forming apparatus according to this embodiment.

According to the execution of programs by the CPU of the sheet finisher 2, the sheet finisher 2 functions as the sheet-information acquiring unit 101, the lifting controller 102, a lowering-amount calculating unit 103A, and the lowering controller 104. In the sheet finisher 2, the respective units 101 to 104 may be provided as hardware. In the second example of the functions of the sheet finisher 2 shown in FIG. 22, units same as those in the first example of the functions of the sheet finisher 2 shown in FIG. 14 are denoted by the same reference numerals and signs and explanation of the units is omitted.

The lowering-amount calculating unit 103A has a function of regarding, if the conversion error ze′ calculated by the above equation (3) is equal to or smaller than a specified value (a positive number), the conversion error ze′ as zero and, on the other hand, if the conversion error ze′ exceeds the specified value, taking into account the conversion error ze′ to calculate the lowering amount (L+ze′) of the stacking tray 23 using the above equation (2).

As explained about the function of the lowering-amount calculating unit 103 shown in FIG. 14, if the conversion error ze′ is taken into account to calculate the lowering amount (L+ze′) of the stacking tray 23 in all cases in which the conversion error ze′ is positive, a lowering amount of the stacking tray 23 by the lowering controller 104 and a lifting amount of the stacking tray 23 by the lifting controller 101 increase and performance falls. Therefore, the lowering-amount calculating unit 103A regards the conversion error ze′ as zero until the conversion error ze′ exceeds the specified value.

The lowering-amount calculating unit 103A sets the conversion error ze′ to a specified value (a ratio to (I+L)) and calculates, using the distance I, the lowering amount L, the sheet thickness, the sheet width coefficient, and the number of sheet bundles subjected to stapling, the number of sheet bundles at which the conversion error ze′ exceeds the specified value for the first time.

For example, the distance I is set to 20 [mm], the lowering amount L is set to 15 [mm], the specified value is set to 28 [mm] (80% of (20+15)), and the thickness of the sheet P is set to 0.1 [mm]. Under these conditions, two A4 plain papers (the sheet width coefficient: 2.2) are stapled. The lowering-amount calculating unit 103A calculates, on the basis of the following formula, the lowering amount (L+ze′) of the stacking tray 23 using the above equation (2) starting from a twelfth sheet bundle.

Number of sheet bundles=28/(1.3−(0.1×2))×2.2=11.57

Ten A4-R plain papers (the sheet width coefficient: 1.1) are stapled under the same conditions. The lowering-amount detecting unit 103A calculates, on the basis of the following formula, the lowering amount (L+ze′) of the stacking tray 23 using the above equation (2) starting from an eighty-fifth sheet bundle.

Number of sheet bundles=28/(1.3−(0.1×10))×1.1=84.84

The lowering-amount calculating unit 103A may calculate a lowering amount of the stacking tray 23 at an interval of plural sheet bundles rather than calculating a lowering amount of the stacking tray 23 using the conversion error ze′ calculated at an interval of one sheet bundle. For example, the lowering-amount calculating unit 103A calculates, for fifty-first to sixtieth sheet bundles, a lowering amount of the stacking tray 23 using the conversion error ze′ calculated at a fifty-first sheet bundle.

FIG. 23 is a flowchart of a second example of the acts of the image forming apparatus according to this embodiment. In the second example of the acts of the image forming apparatus 100 shown in FIG. 23, acts same as those in the first example of the acts of the image forming apparatus 100 shown in FIG. 21 are denoted by the same reference numerals and explanation of the acts is omitted.

The sheet finisher 2 determines whether the conversion error ze′ calculated in Act 3 exceeds the specified value (Act 10). If the sheet finisher 2 determines that the conversion error ze′ exceeds the specified value (“YES” in Act 10), the sheet finisher 2 calculates the lowering amount (L+ze′) of the stacking tray 23 using the above equation (2) (Act 5).

On the other hand, if the sheet finisher 2 determines that the conversion error ze′ is equal to or smaller than the specified value (“NO” in Act 10), the sheet finisher 2 calculates the lowering amount L of the stacking tray 23 using the above equation (1) (Act 6).

As explained above, the image forming apparatus 100, the sheet finisher 2, and the method for driving the stacking tray 23 according to this embodiment can stack a large number of the sheet bundles B subjected to stapling on the stacking tray 23 and stack the sheet bundles B with high alignability. 

1. An image forming apparatus comprising: a printer configured to print image data on a sheet; a stapler configured to staple a sheet bundle, the sheet bundle including the sheet; a stacking tray configured to receive the sheet bundle at a standby position; a driving mechanism configured to lift and lower the stacking tray; a detecting unit configured to detect an upper surface of the sheet bundle on the stacking tray; a storage unit configured to store the number of sheet bundles on the stacking tray; and a controller configured to control the driving mechanism to increase a distance from the detecting unit to the stacking tray at the standby position according to an increase in the number stored in the storage unit.
 2. The apparatus according to claim 1, wherein the storage unit stores the number of sheets included in the sheet bundle, and the controller controls the driving mechanism to reduce the distance according to an increase in the number of sheets included in the sheet bundle stored in the storage unit.
 3. The apparatus according to claim 1, wherein the controller controls the driving mechanism to increase the distance according to an increase in a size of the sheet.
 4. The apparatus according to claim 1, wherein the controller controls the driving mechanism to keep the distance if the number is equal to or smaller than a specified value.
 5. The apparatus according to claim 1, wherein the controller controls the driving mechanism to change the distance corresponding to a thickness of a plurality of sheet bundles.
 6. A sheet finisher comprising: a stapler configured to staple a sheet bundle, the sheet bundle including a sheet; a stacking tray configured to receive the sheet bundle at a standby position; a driving mechanism configured to lift and lower the stacking tray; a detecting unit configured to detect an upper surface of the sheet bundle on the stacking tray; a storage unit configured to store number of sheet bundles on the stacking tray; and a controller configured to control the driving mechanism to increase a distance from the detecting unit to the stacking tray at the standby position according to an increase in the number stored in the storage unit.
 7. The finisher according to claim 6, wherein the storage unit stores the number of sheets included in the sheet bundle, and the controller controls the driving mechanism to reduce the distance according to an increase in the number of sheets included in the sheet bundle stored in the storage unit.
 8. The finisher according to claim 6, wherein the controller controls the driving mechanism to increase the distance according to an increase in a size of the sheet.
 9. The finisher according to claim 6, wherein the controller controls the driving mechanism to keep the distance if the number is equal to or smaller than a specified value.
 10. The finisher according to claim 6, wherein the controller controls the driving mechanism to change the distance corresponding to a thickness of a plurality of sheet bundles.
 11. A method for driving a stacking tray comprising acts of: stapling a sheet bundle, the sheet bundle including a sheet; and controlling driving of a stacking tray to increase a distance from a detecting unit that detects an upper surface of the sheet bundle on the stacking tray to the stacking tray at a standby position according to an increase in number of sheet bundles on the stacking tray.
 12. The method according to claim 11, wherein the act of controlling controls the driving of the stacking tray to reduce the distance according to an increase in the number of sheets included in the sheet bundle.
 13. The method according to claim 11, wherein the act of controlling controls the driving of the stacking tray to increase the distance according to an increase in a size of the sheet.
 14. The method according to claim 11, wherein the act of controlling controls the driving of the stacking tray to keep the distance if the number is equal to or smaller than a specified value.
 15. The method according to claim 11, wherein the act of controlling controls the driving of the stacking tray to change the distance corresponding to a thickness of a plurality of sheet bundles. 